JS at ure^s Operations which Man is competent to Study. 4(>3 



to the place where we insert these numbers we can make them 

 express by how many metres, or by what fraction of a metre, 

 we are to measure any of the magnitudes with which man has 

 become acquainted throughout the whole range of his study of 

 Nature. 



In this table metros mean decimal multiples of the metre ; 

 metrets mean its decimal sub-multiples ; and kilem (to be 

 pronounced with the i long*, as in mile) is used as convenient 

 English for the French " kilometre." The first few places in 

 the table and the last t* our or five lie beyond the range of our 

 present knowledge. Nevertheless they ai'e included ; in order 

 that the table may not be unduly shortened by temporary 

 ignorance on our part, but may provide a large margin for 

 possible future discoveries. 



The significance of the survey is best appreciated by exa- 

 mining separately the four groups into which the table is 

 divided, and it is convenient to begin with Group C, as it 

 includes the measures most familiar to us. 



Group C {Laboratory Measures). 



Group C extends from kilems (kilometres) on the left down 

 to tenths of a micron on the right. The central sub-section v 

 includes the measures most in use in our laboratories, from 

 metres down to tenths of a millim or millimetre. Sub-section u 

 includes those larger measures which men have also in every- 

 day use — from tenths of a metre up to kilems or kilometres. 

 The third sub-section w, from millims (millimetres) down to 

 tenths of a micron, covers the entire range of the microscope, 

 and indeed travels somewhat beyond the grasp of that instru- 

 ment, since the smallest interval at which two objects can be 

 seen as two by the best immersion objectives supplemented by 

 the best immersion condensers, and most carefully handled, is 

 but little less than two tenths of a micron, which is the 

 127,000th of an inch ; whereas sub-section Qw extends twice 

 as far, i. e. down to one tenth of a micron. This brings us 

 within the border of the next group — the group of molecular 

 intervals, almost all of which lie farther beyond the reach of 

 the microscope than microscopic objects lie beyond the grasp 

 of the naked eye. 



Group D (Molecular Quantities). 



On the borderland between groups and D we find the 

 lengths of waves of light, all of which can be represented by 



* In xtXids, " a thousand," and in all Greek words derived from ,)(iAiaf, 

 lie t is long. 



